1. Field of the Invention
The present invention relates to a vehicle roof structure, and more particularly to a vehicle roof structure which can secure a wide space inside a passenger compartment while improving the rigidity thereof.
2. Description of the Related Art
As is known, it is an inevitable problem that the rigidity of a roof portion of a vehicle such as an automobile declines because the roof portion has a relatively wide area.
To cope with this problem, a structure has conventionally been adopted in which reinforcements each having a U-shaped cross section are disposed on a passenger compartment side of a roof panel so that the roof panel is supported from a back side thereof by flange portions formed along side edge portions of the reinforcements. End portions of the reinforcements are supported by roof side rails which supports the sides of the roof portion together with the roof panel, whereby the surface rigidity of the roof portion is enhanced. A structure of this type is disclosed in, for example, JP-A-08-113161.
With the prior art vehicle roof structure, however, the vertical walls of the reinforcements need to be taller in order to increase the bending rigidity of the reinforcements themselves so that the surface rigidity of the entirety of the roof portion is secured by the reinforcements and as a result, there is caused a problem that the vertical dimension of the passenger compartment is reduced by such an extent that the height of the vertical walls is increased to thereby narrow the space inside the passenger compartment.
In addition, it may be considered to increase the thickness of the reinforcements in order to increase the rigidity thereof without increasing the height of the vertical walls, but this leads to another problem that the vehicle weight is increased.
To cope with the problems, an object of the invention is to provide a vehicle roof structure which can increase the rigidity thereof without sacrificing the space inside the passenger compartment.
With a view to attaining the object, according to a first aspect of the invention, there is provided a vehicle roof structure characterized in that a front part of a roof panel (for example, a roof panel 8 in an embodiment) is formed into an arch configuration which declines gradually while a rear part of the roof panel is formed flat and in that stiffeners (for example, stiffeners 9 in the embodiment) each having a U-shaped cross section are provided on the rear part of the roof panel, terminal ends of each stiffener being folded back.
By providing the vehicle roof structure accordingly, it is possible to increase the rigidity of the roof panel at the front part thereof by forming the arch configuration which is advantageous in providing the rigidity and at the flat rear part thereof by the stiffeners whose terminal ends are folded back.
According to a second aspect of the invention, there is provided a vehicle roof structure as set forth in the first aspect of the invention, wherein the stiffeners are disposed on a passenger compartment side of the roof panel in a transverse direction, and wherein end portions (for example, left and right attaching portions 26, 27 in the embodiment) of the stiffeners are attached to vehicle body frames (for example, left and right frames 5, 6 in the embodiment) which support side edges (for example, left and right attaching flanges 14, 15 in the embodiment) of the roof panel.
By providing the vehicle roof structure accordingly, a load exerted on the stiffeners from the roof panel is securely distributed to the vehicle body frame from the stiffeners whose rigidities are secured.
According to a third aspect of the invention, there is provided a vehicle roof structure as set forth in the first or second aspect of the invention, wherein horizontally extending flange portions (for example, flange portions 18 in the embodiment) are formed at upper edges of vertical walls (for example, vertical walls 17 in the embodiment) of each of the stiffeners as roof supporting portions, and wherein folded-back portions (for example, folded-back portions 20 in the embodiment) which are bent along the vertical walls are formed along side edge portions of the flange portions.
By providing the vehicle roof structure accordingly, when the roof panel attempts to deform in an out-of-plane fashion due to vibrations caused while the vehicle is running, while a load is exerted on the stiffeners in a bending direction, the vertical walls and folded-back portions of the stiffeners both receive the load in an inplane direction.